Backhaul for many public access and some enterprise small cell deployments involves a wireless link for the last few hundred metres. It may not be feasible to connect fibre in every case, both because time and cost are prohibitive. One of the choices for that last hop is point-to-point microwave operating at 60GHz (Extremely_high_frequency).
Wireless transmissions at these frequencies are limited by the high oxygen absorption, with a maximum range of 2 kilometres. This matches the requirement for small cell backhaul links of a few hundred metres – typically the range 100 to 700 metres being the sweet spot. The signal won't penetrate buildings or foliage, requiring clear line of sight between transmitters but multiple links can be used to relay the signal around obstructions. By using a parallel uplink and downlink, bi-directional data rates of up to 1Gbit/s are available in commercial products today. Constraining the link data rate to 100Mbit/s or 300Mbit/s allows lower order modulation to be used, either increasing the range or further enhancing link reliability. The short range and narrow beams provide very high reuse, making it ideal for those high traffic urban areas.
The use of such high frequencies require a smaller physical dish antenna, which matches the low profile and form factor needed to satisfy town planning constraints.
Transmissions are affected by rain, which reduces maximum operating range of each link. For the shorter distances of a few hundred metres needed for small cell deployments, five nines (99.999%) uptime is quite achievable.
Licence Exempt Spectrum
Many countries have taken a pragmatic approach to the use of the 60GHz spectrum, assigning it as “Licence Exempt”. This has a slightly different interpretation compared with “Unlicensed”, as is used by Wi-Fi. Since these signals very rarely interfere with each other because they are point-to-point beams, they can co-exist with competitors even in high traffic congested areas. Regulators may require a nominal fee to register, or simply allow unconstrained deployment nationwide.
By comparison, other popular spectrum bands used for microwave links may attract hefty spectrum licence fees. It is not unusual for network operators to pay millions of dollars for the rights to specific bands for nationwide use, equating to thousands (or even tens of thousands) of dollars per link installed.
Short time to deploy
Lower frequencies used for longer range macrocell backhaul typically require frequency planning, co-ordination and approval from the regulatory authorities, which all takes time and expertise to organise.
An on-site survey is mandatory to establish the exact transmission path and ensure ongoing line-of-sight. This requires some expertise, and needs to take account of potential changes to the environment such as foliage which might subsequently grow into the beam-path or pragmatic aspects such as access from the tall building rooftop for window cleaning gantries. Some technical skill is required to setup the equipment, but not nearly to the same level as an RF design specialist. Such point-to-point links should be able to be installed by technicians with a similar skill level as a satellite TV dish installer. In both cases, alignment of the dish to optimise for maximum signal strength using suitable test equipment is essential.
The installation work may be carried out in-house, outsourced or leased depending on the particular circumstances of the network operator and the country/environment they operate in.
Providing high capacity
The high throughput rates of up to 1Gbit/s should more than satisfy the needs of advanced multi-mode small cells. Alternative solutions which share spectrum or resource between a cluster of cells may not be able to match the peak throughput of the radio links they service. This problem also occurs where Wi-Fi or other unlicensed/shared spectrum is used for backhaul, or where licensed spectrum for backhaul use is scarce.
Today's individual small cells may be more than adequately served with a backhaul link capacity of 100Mbit/s. Multiple small cells, daisy chained along a street canyon, may increase that requirement to 300Mbit/s or more. In the future, LTE Advanced and Multi-mode 3G/LTE small cells could reach peak traffic rates of 500Mbit/s to 700Mbit/s, which would be within the 1Gbit/s capacity of the technology.
An advantage of FDD links of this type is low latency, typically sub 50 microsecond, and so this doesn't affect the end-user experience. TDD links on the other hand would naturally add some delay due to the buffering while waiting for the next transmission slot. With LTE offering end-to-end latency in the order of 10's of milliseconds, this may be considered significant.
Network synchronisation can be provided using IEEE 1588 to augment the small cell's own GPS timing. It is also likely that we'll see more SyncE (Synchronous Ethernet) extended through these links in the near future.
Point to point 60GHz microwave links have an important part to play in small cell backhaul for public access and some enterprise deployments. They are one of several options in the toolkit of backhaul solutions (alongside NLoS, point-to-multipoint microwave, fibre, DSL, Cable) which will be used to a different extent by each network operator.
Our thanks to Stuart Broome, CEO of sub10 networks, who provided insight and input for this page.